A power semiconductor module unit has a structure in which a power semiconductor chip such as an IGBT (insulated gate bipolar transistor), a MOSFET (metal oxide semiconductor field effect transistor), or a FWD (free wheeling diode) is solder-bonded to an insulating circuit board having a predetermined circuit, and the power semiconductor chip and the insulating circuit board are sealed by a resin. Such a power semiconductor module unit is used for controlling large current in electric automobiles, electric railroad vehicles, machining tools, and the like, for example. Since a power semiconductor chip in which large current flows radiates a large amount of heat, the semiconductor module unit requires a structure capable of efficiently radiating heat in order to realize a compact outer shape. Moreover, practically, the semiconductor module unit is used in a state of being attached to a cooler in order to further improve the heat radiating performance. In the following description, a structure in which a semiconductor module unit is attached to a cooler will be referred to as a semiconductor module.
Conventionally, in order to radiate heat efficiently, an insulating circuit board in which a plurality of power semiconductor chips which is a heat generation source is solder-bonded is solder-bonded to a metal substrate of copper or the like having an excellent heat conducting property (heat radiation performance). Further, the rear surface of the metal substrate is close-contacted with the cooler with heat-radiating grease interposed and is fixed to the cooler by bolts and nuts inserted through holes formed in the periphery of the metal substrate (Patent Literature 1).
Moreover, the operation of fixing the semiconductor module unit to the cooler is generally performed by the user of the semiconductor module unit. In general, many heat-radiation metal substrates use copper or copper alloys as the main material thereof, and many coolers use aluminum or aluminum alloys as the main material thereof. The heat conductivities of copper and aluminum are approximately 390 W/m·K and approximately 220 W/m·K, respectively. In contrast, since the heat conductivity of heat-radiating grease is as small as approximately 1.0 W/m·K, the heat-radiating grease serves as a large heat resistance component in relation to heat radiation of power semiconductor modules. Thus, the temperature of semiconductor chips is likely to increase, and accordingly, the allowable current capacity decreases.
Recently, there is an increasing interest in energy-saving and clean energy techniques, and wind power generation and electric automobiles are the leading techniques in the market. A power semiconductor module used in these uses handles large current and needs to transfer heat energy generated by power semiconductor chips to a cooler with high efficiency as much as possible to radiate heat to the outside in order to realize a compact outer shape. In particular, in electric automobile, a water cooling method is used as a cooling method in order to further reduce the size and weight. Moreover, a cooler-integrated semiconductor module has already been developed and used practically (Patent Literatures 2, 3, and 4). The cooler-integrated semiconductor module has a structure in which an insulating circuit board having semiconductor chips mounted thereon is directly bonded to the surface of a water-cooled cooler by eliminating the heat-radiating grease serving as a heat resistance component and a thick and heavy metal substrate having a large outer shape. As a method of directly bonding the insulating circuit board having semiconductor chips mounted thereon and the cooler, a bonding method which uses a solder material is generally used. A structure in which a resin frame that surrounds the periphery of the semiconductor chips and the insulating circuit board is fixed to the cooler using bolts is also known (Patent Literature 4).
Patent Literature 1: Japanese Patent Application Publication No. 2008-288414
Patent Literature 2: Japanese Patent Application Publication No. 2008-226920
Patent Literature 3: Japanese Patent Application Publication No. 2012-142465
Patent Literature 4: Japanese Patent Application Publication No. 2008-218814
However, in the method of directly solder-bonding the insulating circuit board having semiconductor chips mounted thereon and the cooler, in order to solder-bond the insulating circuit board to the cooler, it is necessary to heat the cooler having large heat capacity to a melting point (approximately 250° C. to 350° C.) or higher of the solder material. Thus, the time required for the soldering operation increases and the production efficiency decreases. Moreover, when the insulating circuit board is solder-bonded to the cooler using flux, it is necessary to rinse the flux adhering to the heavy and bulky cooler, and the operation efficiency decreases.
Further, after an insulating circuit board having a plurality of semiconductor chips mounted thereon is solder-bonded to a cooler, when the circuits on the semiconductor chips or the insulating circuit board are connected by wire bonding, it is necessary to perform the wire bonding operation in a state in which the cooler is bonded to the insulating circuit board. However, since propagation of ultrasound waves required for realizing wire bonding connection is not likely to be sufficient, bonding defects may occur. Further, if the ultrasound wave power is increased to avoid the bonding defects, the damage to the semiconductor chips may increase.
As described above, the structure in which, after the insulating circuit board is directly bonded to the cooler by eliminating the heat-radiating grease and the heat-radiation metal substrate, the semiconductor chips are bonded to the insulating circuit board in order to reduce the size and weight of the semiconductor module has a problem that the yield rate may decrease and the assembling cost may increase.
Moreover, in a structure in which a resin-sealed semiconductor module unit with no heat-radiation metal substrate is manufactured, and the semiconductor module unit is directly solder-bonded to a cooler to form a semiconductor module, a metal surface on the rear surface of an insulating circuit board exposed to a portion of the bottom surface of the semiconductor module unit is solder-bonded to the surface of the cooler. The area of the soldered portion contacting the cooler, on the bottom surface of the semiconductor module unit is smaller than that of the heat-radiation metal substrate, and the resin portion around the bonding portion is not attached to the cooler. Thus, deterioration is likely to progress from the periphery of the solder bonding portion.
Moreover, since the structure in which the resin frame surrounding the periphery of the semiconductor chips and the insulating circuit board is fixed to the cooler by bolts requires a bolt fastening operation, the assembling cost may increase.